after i leave my plates out for 4-5 days, fungus starts growing on them. is this caused by contamination while pouring them? or is it unavoidable because they were left out a long time. is it caused by some moisture buildup?

no we never pour them in the sterile hood, just next to a flame. i have extras that i end up not using and like one or two out of a stack of ten will start growing mold... i'm not sure why this happens to me. I'm very careful when i pour the plates and immediately cap them. is it something on my hands? i usually don't wear gloves when i pour plates.

sixpence wrote:no we never pour them in the sterile hood, just next to a flame. i have extras that i end up not using and like one or two out of a stack of ten will start growing mold... i'm not sure why this happens to me. I'm very careful when i pour the plates and immediately cap them. is it something on my hands? i usually don't wear gloves when i pour plates.

Hmmm... I think it would make sense to wear gloves. Sometimes i also pour my plates outside the laminar flow. But i always sterilize the whole place and also the gloves i'm wearing- and I never had any contamination.

I think it's better to follow the steps in Laboratory Experiments in Microbiology book written by Ted R. Johnson and Christine L. Case. You'll find some steps to prepare workbench and also steps in pouring media (also with pictures)

Q: Why are chemists great for solving problems?
A: They have all the solutions.

i think that if the growth appered after " or ' days that fungi and if u show that after( or - days tha's actinomycetes , in the two cases , there are two reasons for that1 the plate petri not sterilization or the time of sterilization in autocalved didn't enught 2 when u pour the media it's contimnated

It is worth remembering that air is full of fungal spores. More in some places than in others.

You should pour plates in a clean environment with still air, to minimise the movement of such spores. Also, open each plate only briefly. It is when you open them that spores drift in.

Spores also fall off anything unclean. Washing hands first and drying them well is good practise. Also, if you have clothing with sleeves, make sure the clothes are clean, since fungal spores can shake off those sleeves if they are not clean.

An interesting experiment to try is to check fungal spore numbers in the area you pour your plates. Open the plates to the air for different periods of time, and incubate, to see how much exposure is needed to guarantee infection. A clean environment should permit at least 50% of all plates to remain uncontaminated after 2 minutes exposure.